Composition for surface treatment

ABSTRACT

A coating composition containing a tetraalkyl silicate or a monomeric or oligomeric hydrolysis product thereof, present in a proportion of 40-90% by weight on the non-volatile content of the composition, and a hydrous oxide sol selected from Type A sols and Type B sols, present in a concentration such that the said oxide constitutes 10-60% by weight of the non-volatile content of the coating composition; optionally together with a surface wetting agent, marker material, defoamer, stabilizer, corrosion inhibitor and/or a catalyst which assists in chain-extension and cross-linking of the hydrolysis products in the presence of moisture; in dispersion in a fluid aqueous medium which is water optionally containing also from 0.1-40% by volume of polar organic liquid at least partly miscible with water. A method of pretreating a metal workpiece by applying this coating composition to a surface of the workpiece and drying the coating. The product is a metal workpiece carrying a surface coating thus formed. Coated workpieces may be secured together by an adhesive to constitute a structure.

This invention relates to aqueous compositions for surface treatment.One purpose of the treatment is to increase the adhesion of subsequentlyapplied coatings, such as paint, lacquer, varnish or adhesive. Althoughthe invention is of application to solid surfaces generally, it is ofmajor importance in relation to metals generally, and in particular toaluminium.

Polydiethoxysiloxanes are used as refractory binders for metal castingmoulds, and as binders for zinc-rich primers. They are made by thecontrolled hydrolysis of tetraethyl silicate to a 40% hydrolysed productknown as "Ethyl Silicate 40" or "Silicate 40". This is a mixture oftetraethyl silicate and various polydiethoxysiloxane oligomers. Theseare generally used in solution in organic solvent; but such solventnecessarily carries a fire hazard.

WO 92/15650 (Alcan) describes an aqueous composition for the treatmentof surfaces such as metal particularly aluminium, comprising atetraalkyl silicate or a monomeric or oligomeric hydrolysis productthereof, and an inorganic passenger powder below 100 nm diameter such asfumed silica, both in dispersion in a fluid aqueous medium. Thecomposition can be applied as a pretreatment coating to improve theadhesion of subsequently applied paint, lacquer or adhesive. Non-aqueousintermediate products comprising the silicate component and passengerpowder can be diluted with water to make the coating composition.

This invention arises from our discovery that the nature of theinorganic component may be critical, and that advantages may result fromreplacing it. In particular, storage stability may be improved.

In one aspect this invention provides a coating composition comprising atetraalkyl silicate or a monomeric or oligomeric hydrolysis productthereof, present in a proportion of 40-90% by weight on the non-volatilecontent of the composition, and a hydrous oxide sol, present in aconcentration such that the oxide constitutes 10-60% by weight of thenon-volatile content of the coating composition, optionally togetherwith a surface wetting agent, marker material, defoamer, stabiliser,corrosion inhibitor and/or a catalyst which assists in chain-extensionand cross-linking of the hydrolysis products in the presence ofmoisture, in dispersion in a fluid aqueous medium.

This coating composition is suitable for the pretreatment of solidsurfaces such as metals generally, including steel, titanium, copper,zinc and, particularly aluminium, which term is used herein to includethe pure metal and its alloys. The composition improves the adhesionproperties of the pretreated surface, by improving the adhesion theretoof a subsequently applied coating such as paint, varnish, lacquer; or ofadhesive either in the presence or absence of a lubricant. Thepretreatment may improve either the initial adhesion of the subsequentlyapplied coating to the surface or the maintenance of such adhesiveproperties in service, or both the initial adhesion and maintenance ofadhesive properties. For example, so far as subsequently appliedadhesive is concerned, the benefits of the invention may be shown mainlynot in the initial adhesive strength obtained, but in the maintenance ofadhesive strength in hostile or corrosive environments.

Tetraalkyl silicates (or tetraalkoxy silanes) can be progressivelyhydrolysed to silica according to the following equation:

    Si(OR).sub.4 →Si(OR).sub.3 --O--Si(OR).sub.3 →.brket open-st.Si(OR).sub.2 O.brket close-st.→.brket open-st.Si(OH).sub.2 O.brket close-st.→SiO.sub.2

Preferably R in these formulae is ethyl. The unhydrolysedtetraalkylester can be used for this invention, and is preferred, but apartially hydrolysed product is a possible alternative. This componentshould generally be present in a proportion of 40-90%, preferably 50-80%by weight of the non-volatile content of the composition.

As hydrolysis proceeds, a starting solution of tetraalkyl silicatethickens and finally sets-up so as to be unusable. The oligomericpartial hydrolysis products that may be present in the compositions ofthis invention are of such size or such concentrations that thecomposition retains a usable application consistency.

The hydrous oxide sol is a stable, aqueous, colloidal dispersioncontaining primary particles or aggregates of primary particles whichare smaller than 150 nm. Depending on the nature of the basic colloidalunit, sols can be classified into three types: type A, B and C.

Type A sols consist of basic units which are polynuclear ions which forman `inorganic polymer` and are formed by hydrolysis and polymerisationof monomeric cations. The molecular weight of the polynuclear cationswill depend on the degree of hydrolysis but these sols normally have ananion to metal ratio of approximately 1:1. The polymeric species are notlarge enough to scatter light efficiently, so the sol and the resultantgel are optically clear. The gel has a high density, low porosity andthe X-ray diffraction pattern consists of very broad bands. J. D. F.Ramsay "Neutron and Light Scattering Studies of Aqueous Solutions ofPolynuclear Ions, Water and Aqueous Solutions", 207-218 1986 (ed G. W.Neilson and J. E. Enderby: Bristol. Adam Hilger). Type A sols may beformed from the polynuclear ions listed in this paper including thosecontaining Al(III) Fe(III) Zr(IV) Th(IV): for example:

    Al.sub.13 O.sub.4 (OH).sub.24 (H.sub.2 O).sub.12.sup.7+.

Type B sols consist of basic units with a definite shape, e.g.spherical, rod or plate-like, and are amorphous or microcrystalline. Thesol is formed by extensive hydrolysis of a salt and has a low anion tometal atom ratio of approximately 0.3:1. The sols can also be preparedby peptization of fresh precipitates. The colloidal units are notaggregated and the sol and the resultant gel are both clear. Type B solsinclude colloidal silica and Al(III) Zr(IV) Ce(IV) Ti(IV) Fe(III).Preparation of Type B Al(III) sols is described in GB 1,174,648.Preparation of Ce Type B sol is described in GB 1,342,893. Type BAlumina Sols are available commercially.

In the type C sol the basic colloidal units are aggregated. They arecrystalline and the gels formed by removal of water and have a lowdensity. these sols scatter light and are therefore opaque. The solsformed from ultrafine powders prepared by vapour phase techniques, i.e.flame hydrolysed powders, belong to this category.

Type A and B sols when dehydrated yield gels which are >45% of thetheoretical density of the oxide. The gels derived from type C sol areporous and have a density <45% of the theoretical density of the oxide.

The inorganic sol for use in this invention is a hydrous oxide sol, thatis to say a Type A or Type B (but not Type C) sol. Examples are zirconiasols, ceria sols, titania sols, hafnia sols, alumina sols, ironoxyhydroxide sols, and particularly colloidal silica or silica sol.

The hydrous oxide sol improves the storage stability of the aqueouscomposition and assists in the formation of an insoluble matrix afterdrying. The hydrous oxide sol is preferably present in the coatingcomposition in a concentration such that the oxide (metal oxide orpreferably silica) constitutes 10-60%, particularly 20-50%, by weight ofthe non-volatile content of the coating composition.

The aqueous medium preferably also contains a co-solvent, which helps towet the surfaces to be coated and also to stabilise the aqueousdispersion. Suitable are polar organic liquids which are at least partlywater soluble and preferably volatile, for example mono and polyhydricalcohols and ethers. When used, the co-solvent is preferably present ina proportion of 0.1-40%, particularly 1-20%, by volume on the volume ofthe whole aqueous medium.

A corrosion-inhibiting constituent may be included, for example azirco-aluminate or a chromate or organo-metallic trivalent chromiumcompound, in proportions such as those typically used in metal coatingcompositions.

The composition may also include a catalyst which assists in thechain-extension and cross-linking of the hydrolysis product in thepresence of moisture. The catalyst is used in a concentration to achievehydrolysis of the tetraalkyl silicate in a convenient time. Suitablecatalysts are mineral and organic acids such as nitric and acetic acidsor bases such as amines. Free bases are not preferred as they produceformulations which are relatively unstable. Other suitable catalysts arelatent catalysts such as salts or complexes of acids and bases whichcease to be neutral on their decomposition. An example of a latentcatalyst which becomes acidic on decomposition is ammonium acetate.Other suitable catalysts are those which are latent and generate baseson decomposition. An example of a blocked catalyst of this kind is anamine titanate chelate sold under the Trade name Tilcom AT 31 andbelieved to have the structure I; when the amine is chelated itsbasicity is reduced. This effect may be enhanced if the amine titanateis further neutralised with acetic acid. ##STR1##

Acid peptised sols, such as a zirconium oxide sol peptised in nitricacid, may also be used.

Catalysts are in general not required in the composition of thisinvention and are preferably omitted. Though also not preferred, it ispossible according to the invention to apply a coating composition notcontaining a catalyst to a surface, and then to apply the catalyst in asubsequent operation to hasten hydrolysis and chain extension of thepolysiloxane.

The composition may also include hydrogen peroxide e.g. at aconcentration up to 20 g/l. This may be particularly useful when an acidcatalyst is used.

Water has a harmful effect on adhesive bonds. Thus components which arehygroscopic or humectant and which act to attract or retain moisture inthe coating, are preferably not included in the coating composition.

Other components are usual in pretreatment coating compositions of thiskind and may be included in conventional amounts in coating compositionsaccording to this invention. Such components include surface wettingagents, marker materials, defoamers and stabilisers. Other conventionaladditives include silanes and organosiloxanes (which contain Si--C bondsand are distinguished on that account from tetraalkyl silicates).

One marker material, useful to indicate the presence or thickness of asurface coating, is polybutyl titanate. The addition of polybutyltitanate to aqueous solutions normally results in the rapid hydrolysisof the titanate with the formation of hydrated titanium dioxide, usuallyas a substantial precipitate, not suitable for roll-coat application.

However, the addition of 10% or even 20% w/w polybutyl titanate to thetetraalkyl silicate contained in a formulation is possible. A stableworking solution, suitable for coil coating, is obtained. This may beachieved by adding the polybutyl titanate to the tetraalkyl silicate, inthe desired amount, before proceeding with completion of theformulation.

A passenger powder of primary particle size below 100 nm may be includedin the coating composition but is not preferred. Passenger powders oftencause formulations to set-up or solidify and always gives rise tosettling problems, and the primary particles aggregate and often formlumps. Passenger powders are not used as matrix builders or to controlthe structural topography of the coatings of this invention, and arepreferably not used at all. However they may be used for secondarypurposes such as markers, in concentration less than 20% or preferably10% of the non-volatile content of the composition.

The coating composition preferably has a pH from 0 to 12, e.g. 1 to 4and particularly from 1.3-3.0 in the presence of an acid catalyst and5-12 e.g. 7-11 in the absence of such a catalyst. If the pH is too high,the emulsion is less stable and the composition has a poor shelf life.If the pH is too low, it may chemically attack the surface to which itis applied. Preferred silica sols are stabilised by alkali which mayalso enhance the stability to hydrolysis of the tetraalkyl silicate.

Tetraalkyl silicates and their partial hydrolysis products are not verystable in neat form, and are generally sold commercially in solution ordispersion in a polar volatile water-miscible organic solvent. Inpreparing coating compositions according to this invention, a convenientfirst step is to disperse the hydrous oxide e.g. silica sol in thesilicate component, which may readily be done by means of a high shearstirrer. A co-solvent, if used, may suitably be added also at thisstage.

The coating compositions of this invention may be supplied in aconcentrated form to which a sufficient volume of a fluid aqueous mediumis added to give a convenient application concentration. The aqueousmedium may be water, optionally containing any desired catalyst and anyhydrogen peroxide. Other components would generally be included in thesupplied concentrated product. A co-solvent may also be used with thissystem. The aqueous coating compositions have a surprisingly good shelflife, of the order of many months. Improved solution stability leads toimproved consistency; the coating compositions of this invention areeasier to prepare on a commercial scale than the compositions ofWO92/15650.

A metal workpiece to whose surface the aqueous composition is to beapplied may be cleaned by conventional means appropriate to thesubstrate concerned. For aluminium this may be an acid or alkalinecleaning treatment, using commercially available chemicals such as thosesold by ICI under the Trademark Ridolene 124/120E or Ridolene 336.

The composition may be applied to the surface by a convenientapplication technique such as roller coating, brushing or spraying. Foraluminium strip, roller coating is likely to be an attractive option.The formulation may need to be adjusted to provide a convenientconcentration for application by the desired method. After application,the coating on the surface is usually dried. Drying temperatures arefrom ambient up to 400° C., and may typically be in the range of50°-200° C. With aluminium substrates, preferred drying temperatures arein the range 100°-150° C.

The surface of the workpiece preferably carries the coating at athickness of 0.005-0.7 μm particularly from 0.01-0.5 μm. It is difficultto apply thicker films satisfactorily by a no-rinse type treatment. Inpractice it is more usual to measure coating weight, and this ispreferably 20-500 mg, e.g. 50-200 mg, per square meter of coatedsurface. The invention envisages as an additional method step theapplication to the coated surface of an organic coating such as paint,lacquer, varnish or adhesive. There is increasing interest in the use ofadhesively bonded aluminium workpieces as structures.

According to another aspect, the invention provides a metal workpieceprovided with a surface coating as described and a lubricant, e.g. presslubricant, on the surface coating. Adhesives which are compatible withlubricants are commercially available. The invention also provides astructure of such metal workpieces secured together by adhesive. Thetechnology is described, albeit with different surface coatings, in U.S.Pat. No. 5,139,888.

A metal workpiece is metal whose shape is not material to the invention,conveniently metal sheet or strip either before or after forming intoshaped components to be adhesively bonded together to form a structure.

There follows a list of commercially available components suitable foruse in the coating compositions of the invention.

Silester OS is the trade mark of a product marketed by Huels, which is amixture of ethyl silicates containing approximately 20% silicon andapproximately 1% ethyl alcohol.

Silester AR is the trade mark of a range of products marketed by Huelswhich are pre-hydrolysed ethyl silicate hybrid binders.

Syton X30 is a trade mark of Monsanto in respect of an aqueousdispersion of very small silica particles, typically 1.20 specificgravity and pH value 10.25 and containing nominally 30% by weight ofsilica. Sytons are slightly opalescent or milky white type B sols inwhich the specific surface area of the SiO₂ is 250 m² /g, and the pH is9.9.

Ludox is the trade mark of a range of silica sols marketed by DuPont andcontaining 30-50 wt % SiO₂ in the form of particles having a specificsurface area of 140-360 m² /g. These also are type B sols. (Ludox AM, pH8.8; Ludox LS, pH 8.1).

Byk-346 is the trade mark of a solution of a polyether modified dimethylpolysiloxane containing 46% non-volatile matter marketed by Byk-Chemieand useful as a surface wetting agent.

Aerosil 380 is the trade mark of Degussa in respect of a fumed silicaused as an inorganic passenger powder.

EXAMPLE 1

Three typical formulations, A being a fumed silica formulation accordingto WO 92/15650, B being a colloidal silica formulation according to thepresent invention, and C being a silica-free formulation, are set outbelow (component parts are by weight).

    ______________________________________    A. Fumed Silica Formulation    Silester OS               375.8    Aerosil 380               179.7    Propane-1,2-diol          242.4    Water                     1212.1    TOTAL                     2000    B. Colloidal Silica Formulation    Silester OS               315.5    Syton X30                 427.5    Propane-1,2-diol          203.6    Byk 346                   15.3    Water                     1038.1    TOTAL                     2000    pH                        9.9    C. Silica-free pretreatment formulation    Silester XAR              330    Dipropylene glycol monomethyl ether                              700    ______________________________________

Typical stability of the above solutions, in terms of freshly made upsolutions kept under typical ambient indoor conditions (i.e. about 20°C.), are:

Fumed Silica formulation A: sets to firm gel in less than two days.

Colloidal Silica formulation B; remains useable for more than sixmonths.

For stability reasons, formulation C could not be produced as awater-based treatment.

EXAMPLE 2

An experiment was performed to evaluate the three formulations ofExample 1. Samples of the aluminium alloy were cleaned by a Ridolene124/120E dip at 60° C. for one minute followed by a three minute dry at100° C. Each of the formulations A, B and C was applied to a cleanedsample by roll coating, and the coated samples dried at 130° C. forthree minutes. In some cases lubricant was applied to thethus-pretreated samples; in other cases lubricant was omitted. Anadhesive was applied to the pretreated samples and used to join themtogether. Adhesive cure conditions were 145°/15 min followed by 190°C./15 min. Neutral salt spray results were obtained using single lapjoints prepared from 2 mm gauge 5754-HO alloy, with a 20 mm×10 mmoverlap. Bondline thickness was 0.2 mm. Three 4 mm holes were drilledacross the centre of the joint after curing, as an accelerating factor,to allow more moisture to enter the joint. Joints were exposed to aconstant high humidity at 43° C. with 5% sodium chloride spray for setperiods of time. After 8 weeks and 20 weeks three replicate joints wereremoved. Residual strength of each joint was measured and compared withunexposed specimens (0 weeks exposure). The results are set out in thefollowing Table 1.

                  TABLE 1    ______________________________________              Neutral Salt Spray Results (Mpa)    Formulation 0 Weeks    8 Weeks    20 Weeks    ______________________________________    A + lubricant                25.6       18.1       16.2    B + lubricant                25.1       19.2       12.8    A no lubricant                25.7       17.6       8.5    B no lubricant                25.5       18.8       13.2    C + lubricant                15.0       7.8        4.8    C no lubricant                15.8       10.9       3.0    ______________________________________

The performance of formulation B is fully as satisfactory as that offormulation A and much better than that of formulation C.

In stress/humidity tests, a string of 6 joints is placed under load at asuitable level (in the example: 5 MPa which represents approximately 20%of failure load) by means of a compressed spring in a stressing tube.The joints are then placed in the stress/humidity cabinet and aresubjected to condensing (100%) humidity with cycling temperature rangeof 42° to 48° C. whilst under constant load. The number of days tofailure of the first joint is recorded. This is then removed andreplaced by a solid aluminium test piece and the string re-loaded. Thisis repeated until three joints in the string have failed. A systemhaving unsatisfactory performance would be expected to fail after lessthan or approximately 100 days. Results for four of the six systems areshown in Table 2 below. These four systems have survived more than 500days without failure, indicating that the new formulations exhibit verygood performance.

                  TABLE 2    ______________________________________    Formulation Days on Test to Date                                No of Failures    ______________________________________    A + lubricant                573             none    B + lubricant                573             none    A no lubricant                573             none    B no lubricant                573             none    ______________________________________

The system "C+lubricant" and "C no lubricant" both failed after about 80days,

EXAMPLES 3 TO 6

The formulations of four further coating compositions according to theinvention are set out in the following Table 3.

                  TABLE 3    ______________________________________                   Example    Component        3       4       5     6    ______________________________________    Silester OS      310     310     310   310    Syton X30        470     470     420   470    Propane-1-2-diol                 200   200    Dipropylene glycol ether 200    Tripropylene glycol ether                     220    Monopropylene glycol ether       200    Water            1050    550     800   550    BYK-346                  15            15    Polybutyl titanate               62    ______________________________________

The formulations described in Examples 3 to 6 are ready to use, and donot require dilution.

Examples 3, 4 and 5 illustrate that a range of propylene glycol ethersmay be used in the formulation. Example 5 is a variant on the "standard"formulation with monopropylene glycol monomethyl ether. Example 4 usesdipropylene glycol monomethyl ether and example 3 uses tripropyleneglycol monomethyl ether. All three formulations give good solutionstorage stability and satisfactory application to aluminium sheet byroll coating. Examples 4 and 5 may be dried at the standard dryingconditions of 3 minutes at 130° C. As tripropylene glycol monomethylether has a higher boiling point (242.4° C.) example 3 does not give adry pretreatment under these conditions. Example 5 is a polybutyltitanate modified formulation. It also has good stability andapplication properties.

Examples 3 and 4 were prepared only to evaluate their stability andapplication. Performance data for examples 5 and 6 are shown below inTable 4.

                  TABLE 4    ______________________________________    Stress/humidity   Neutral salt spray results (Mpa)    Example at 5 Mpa load 0 weeks  8 Weeks                                          20 weeks    ______________________________________    5       >650 days     23.9     19.5   16.5            (no failures)    6       >650 days     24.6     19.1   12.8            (no failures)    ______________________________________

EXAMPLES 7 TO 9 Formulation of "Non-Silica" Examples are Shown in Table5

All components are by weight:

                  TABLE 5    ______________________________________    Example       7         8         9    Component     Zr Example                            Example   Al Example    ______________________________________    Silester OS (Huls)                  310       310       310    Bacote 20     310    Polybutyl titanate      124    Aluminium C                       260    Propane-1,2-diol                  200       200       200    BYK 346       15        15        15    Water         to 2000 milliliters in each case    pH (after 6 weeks                  7.8       2.4       1.4    storage at 20° C.)    Initial Strength (MPa)                  25.7      27.6      27.6    ______________________________________

Bacote 20 is a zirconia sol from Magnesium Elektron, which has beenmodified by addition of 10% zirconium nitrate hydrate.

Polybutyl titanate is a titania sol from BTP Tioxide.

Aluminium C is an alumina sol from Degussa.

Initial bond strength was determined as in Example 2.

Both the titania and alumina pretreatments have an acid pH. Allformulations were stable although there was some increased viscosity andsettling. Shaking restored them to a homogenous dispersed state.

EXAMPLE 10 Commercial Trial

A full plant trial was carried out to assess the scale-up, applicationand performance of the pretreatment. 400 liters of pretreatment wereprepared using U.S. sourced equivalents of formulation "B" of Example 1.

    ______________________________________    Formulation:    ______________________________________    Dynasil 40 (Huls - equivalent to Silester OS)                            420    Ludox HS30 (Dupont equivalent to Syton X30)                            310    Propane-1,2-diol        200    BYK 346                 15    Water                   1020    Dynasil 40 is stated to have an average molecular weight of    610.    ______________________________________

The trial was carried out on a full production line using 2 mm gaugealuminium coils (AA5754) with a width of 1700 mm.

On applying the pretreatment solution to pick up and applicator rolls,wetting occurred immediately (current product requires some conditioningof the rolls). Application to the sheet was very uniform and gave thecorrect pretreatment coat weights (50-100 mg/m²). Post-trial clean upwas very simple. The line crew was very positive toward thispretreatment compared to the current product.

Performance data on material from this trial is shown in Table 6.

                  TABLE 6    ______________________________________             Stress/humidity                          Neutral salt spray results (Mpa)    Example  at 5 Mpa load                          0 weeks  8 Weeks                                          20 weeks    ______________________________________    Pretreatment             >132 days    25.1     17.9   16.6    + Lubricant             (no failures)    Pretreatment             >132 days    25.5     20.0   18.3    without  (no failures)    Lubricant    ______________________________________

I claim:
 1. A coating composition having a non-volatile content andcomprising a tetraalkyl silicate or a monomeric or oligomeric hydrolysisproduct thereof, present in a proportion of 40-90% by weight on thenon-volatile content of the composition, and a hydrous oxide solselected from Type A sols and Type B sols, present in a concentrationsuch that the oxide of the hydrous oxide sol constitutes 10-60% byweight of the non-volatile content of the coating composition;optionally together with at least one of a surface wetting agent, amarker material, a defoamer, a stabilizer, a corrosion inhibitor and acatalyst which assists in chain-extension and cross-linking of themonomeric or oligomeric hydrolysis product; in dispersion in a fluidaqueous medium which is water optionally containing also from 0.1-40% byvolume of polar organic liquid at least partly miscible with water.
 2. Acomposition as claimed in claim 1, wherein the oligomeric hydrolysisproduct is a polysiloxane.
 3. A composition as claimed in claim 1,wherein the tetraalkyl silicate or monomeric or oligomeric hydrolysisproduct thereof is tetraethyl silicate.
 4. A composition as claimed inclaim 1, wherein the hydrous oxide sol is a silica sol.
 5. A method ofpretreating a metal workpiece, which method comprises applying to asurface of the workpiece a coating composition according to claim 1, anddrying the coating.
 6. A metal workpiece having a surface coating formedby applying to the surface of the workpiece a coating composition anddrying the coating; wherein the coating composition has a non-volatilecontent and comprises a tetraalkyl silicate or a monomeric or oligomerichydrolysis product thereof, present in a proportion of 40-90% by weighton the non-volatile content of the composition, and a hydrous oxide solselected from Type A sols and Type B sols, present in a concentrationsuch that the oxide of the hydrous oxide sol constitutes 10-60% byweight of the non-volatile content of the coating composition;optionally together with at least one of a surface wetting agent, amarker material, a defoamer, a stabilizer, a corrosion inhibitor and acatalyst which assists in chain-extension and cross-linking of themonomeric or oligomeric hydrolysis product; in dispersion in a fluidaqueous medium which is water optionally containing also from 0.1-40% byvolume of polar organic liquid at least partly miscible with water.
 7. Ametal workpiece as claimed in claim 6, wherein a lubricant is present onthe surface coating.
 8. A metal workpiece as claimed in claim 6, whereinthere is present an organic film overlying the surface coating.
 9. Ametal workpiece as claimed in claim 8, wherein the organic film is apaint lacquer or varnish film.
 10. A metal workpiece as claimed in claim6, wherein there is present an adhesive overlying the surface coating.11. A structure of metal workpieces secured together by adhesive;wherein each metal workpiece has a surface coating formed by applying tothe surface of the workpiece a coating composition and drying thecoating; wherein the coating composition has a non-volatile content andcomprises a tetraalkyl silicate or a monomeric or oligomeric hydrolysisproduct thereof, present in a proportion of 40-90% by weight on thenon-volatile content of the composition, and a hydrous oxide solselected from Type A sols and Type B sols, present in a concentrationsuch that the oxide of the hydrous oxide sol constitutes 10-60% byweight of the non-volatile content of the coating composition;optionally together with at least one of a surface wetting agent, amarker material, a defoamer, a stabilizer, a corrosion inhibitor and acatalyst which assists in chain-extension and cross-linking of themonomeric or oligomeric hydrolysis product; in dispersion in a fluidaqueous medium which is water optionally containing also from 0.1-40% byvolume of polar organic liquid at least partly miscible with water.